Element points to foreign doc & schema

Hi,

Is the following possible?
In my schema, I need an element to be of a type that is defined outside of the current schema.  This should be easy enough be using <import> or <include>.

However, what I really want is for this elemnt to be a pointer (an <XPoinnter>?) to another resource.  Thus, in my schema, I want to be able to define an element such that:

a) it allows for the element to point to an outside resource/instance
b) the schema for this instance is foreign to the current schema that defines this pointner element, but the pointer element forces use of that schema

Is this possible?  

Thanks,

Ryan